Radioreceiving circuits



Dec. 17, 1929. R K- P01-TER 1,739,520

RADIORECEIVING CIRCUITS Filed Dec. 4. 1926 elector 05 l'n't I Signal I i g2 h INVENTOR i /.Yote/f I etectar and lng/0Q2# A TTOPNE Y Patented Dec. 17, 1929 UNITED STATES PATENT OFFICE RALPH K. POTTER, 0F NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK RADIORECIVING CIRCUITS Application led December 4, 192B. Serial No. 152,659.

This invention relates to radio receiving circuits, and more particularly to methods of and means for overcoming fading effects in radio reception.

It has been discovered that in night-time transmission of radio signals, at least at the wave lengths used in broadcasting, the signals sent from a point ot transmission to any point of reception follow at least two major paths. One of these paths is a direct path between the transmitter and the receiver, while the otherl path is devious and apparently of considerably greater length than the direct path. It appears further that these two major paths are in a common vertical plane between the transmitter and the receiver, and that the direct and indirect signal energy components received at the distant point combine at a space angle in this plane.

It is the object of my invention to evercome the fading eilects due to the existence of these two major transmission paths, and thus to provide for satisfactory reception during the periods when such fading is observed with the ordinary receiving system.

In general, I accomplish the desired re sult by establishing two reyeiving points in a common vertical plane with the point of transmission, providing for a suitable space relation between the two points ot inception, and combining the directly received signal and the indirectly received signal iu such phase relation that one or the other is cancelled.

My invention will be clearlyv understood when the following detailed description is read with reference to the accompanyingr drawing, in which- Figure 1 illustrates schematically' the condition of transmission in which the signals follow both a direct path and an indirect path between the point of transmission and a point of reception and indicates one desirable position of the two receivers;

Figure 2 shows diagrammatically one arrangement of the receivers and associated circuits;

Figure 3 comprises two vector diagrams indicating the phase relations of Figure 2;

Figure 4 shows diagrammatically a second arrangement of the receivers and associated circuits;

Figure 5 shows by means of vector dia grams the phase relations of Figure 4, and

Figure 6 shows diagrammatically a third arrangement of the receivers and associated circuits.

Figure l of the drawing illustrates schematically and approximately the condition of night-time transmission discussed above. It two radio receivers R1 and R2 are placed in a common vertical plane with the transmitter T, the two receivers having a considerable horizontal separation, radio signals sent from T will reach the receivers R1 and R2 over a direct path DP. Also, the signals will reach the receiver R1 over some such path as IP1 and the receiver R2 over some such path as 1PZ.- The horizontal separation of the two receivers may be made such that the signals received at the two points over either the direct or the indirect path will be completely out of phase and may be cancelled in a common output ot the two receivers by adjusting the amplification oi' each to give royal ouput, while the signals received over the other path may be conibined either completely or partij.' in phase.

The first arrangement which will be corsidcred in detail is one in which the rez'eivers are so separated that the directly received signals will be combined absolutelyv in phase. while the indirectly received signals will have a phase difference of 180O and may be canreici. iiih 1i fl'ven Wave length i the above condition is satisfied it the receiver outputs are equal and if Letting a equal the angle of incidence ot the indirect wave, it is apparent that a: b cos a (2) Substituting (2) in (1),we find that )cl2 Feast (3) Hence, if the value of the angle a is known, it is possible to determine accurately the necessary spacing of the two receivers Rl and R2, to suppress either the directly or indirectly transmitted wave.

l ith a wave length of 492 meters, it has been determined that the difference in length between the direct path and the indirect path is approximately 135 kilometers for a distance of 110 kilometers between transmitter and receiver. It is assumed that the direction of the indirect path is caused either by reiection from an overhead layer or by refraction. Accordingly, with the data stated above, it is found that the value of the angle a is approximately 26. Substituting in (3), we now find that for the condition under consideration the receivers R1 and R2 must .be separated by distance of approainiately two and one-half kilometers. lt is to be noted that the value of .7) (the distance of separation of the receivers) varies directly with a. Thus it is readily seen that while the separation in the speciiic case considered above is very great, for the shorter wave lengths this distance is much smaller and the arrangement discussed becomes a practicable one.

Fig. 2 of the drawing shows diagrammatically a desirable arrangement of two receivers employed in radio reception based upon the principle discussed above. The circuit comprises the antenna A1 and the detecting and amplifying apparatus R1, the antenna A2 and the detecting and amplifying apparatus R2, the coil C and detecting and amplifying means between the secondary winding of the coil C and the output of the system. If the receivers are so separated that the directly received signals, for instance, are in phase, and the indirectly received signals are completely out of phase and of equal amplitude, it is readily understood that given an accurate determination of the desired spacing of the receivers, the directly received signals will combine in the coil C and will be passed to the output, while the indirectly received signals will be cancelled out. ln this case, however, dissimilarities in the receivers may call for the placing of a phase shifter in the branch of one of the two receivers in order that slight phase discrepancies may be taken care of. A beating oscillator may be employed to reduce the frequency of both branches of the system to a value which is more easily handled than the received frequency without changing the phase relation. This oscillator will function to impress the beating frequency upon the two antennal A1 and A2 with or without an antenna. The antenna shown in the ;lra.ving serves to bring out clearly the idea that the beating oscillator radiates its oscillations.

Satisfactory results may be obtained with the arrangement shown in Fig. 2 by providing a smaller separation than that shown in Fig. l. For instance, the receivers may be separated by a distance equal to one-half the wave length received. The result may now be that the directly received signals will be equal in amplitude and have a phase difference in the two paths of i800, while the in directly received signals will not add in phase, but will have a vector sum depending upon the relation of the distances al and b1. Letting D repreesnt the distance between the receivers, and a the angle of incidence of the indirect wave, the two signals will have a phase separation equal to 2WD(l-Sin a) lf the indirect signals are to be cancelled and the direct signals used, no shifting of phase is required. However, the direct signals may be used and the indirect signals cancelled, by the inclusion of the phase shifter in one of the receiving branches. The phase relations will be more clearly understood upon examination of Fig. 3 of the drawing, which shows a pair of vector diagrams. The phase of the directly received signal in the antenna A1 is represented by the vector Edl, and the indirectly received signal is represented, with respect to phase, by the vector Eil in the upper diagram. The vectors Edz and E@ of the lower diagram represent the signals received in the antenna A2 directly and indirectly, respectively. If it is desired to cancel out the signals indirectly received and to use the directly receivedsignals, the phase shifter is employed in the branch of receiver R2 to shift the phase of the signals received in R2 as shown in the upper diagram. The indirectly received signals are equal in amplitude and are given a phase difference of 1800 and may accordingly be cancelled, while the directly received signals having a phase difference less than 1800 will have a positive resultant value. The signal impressed upon the output is then represented by the resultant vector 'Er which is the sum of the vectors Edi and Edg of the upper diagram, that is, the vector sum after the phase has been shifted in the second receiver. Of course, with the use of a phase shifter to change the phase of the signals received in one receiving branch, the two receivers may be separated by any suitable distance. In such a case, the phase is shifted so that when the signals of equal amplitude are combined, either the direct or the indirect signals will have a phase difference of 1800 and will be cancelled.

lt is to be understood that vertical antenna may be employed in place of the loops shown in the drawing. Such a change does not disturb the phase relations, the determining reiation being that of the distances a., and .7), representing the difference in time phase ofthe signals arriving at the two points of reception.

A modified arrangement of two receiversl suitably spaced in a common vertical plane with the transmitter is shown in Fig. 4 ot the drawing. The antennae A3 and A4 are spaced vertically in the plane of transmission. ll'vith the arrangen'ient shown in Fig. 4, the direct waves will arrive in time phase, while the indirect waves will have a phase diference determined by the distance 91. In this case, no phase shii'ter is required, but the outputs of the intermediate frequency amplifiers are.` reversed by a suitable and obvious arrangement ol the windings lV, and NVZ of the coil C1. The result is that the directly received signals are cancelled, and the indirectly received signals are combined with a phase difference, the output being the vector sum of the indirectly received signals. Fig. shows a pair of vector diagrams helping to clarity the phase relations involved in the arrangement of Fig. 4. The vectors Ecll and Fil represent the directly and indirectly received signals, respectively', of the first receiver', while the vectors ECZ2 and Eig of the right diagram represent the phase relation ot' the signals received in the second receiver. The eii'ect of the reversal ot the equalized outputs ot' the intermediate frequency amplitiers of Fig. 4 is shown in the let't diagram of Fig. 5, the broken vector Edg cancelling the vector Fall, and the position of the Vector Fig separated from Edz by the angle 9 (cor 1 jpending to the distance 91 of Fig. 4) del'icrmining with Eil the resultant vector A. modification ot the arrangement of Fig. 4 is shown in Fig. G of the drawing. In this case, the two antennae A and A6 are so disposed in the vertical plane that the indirect waves arrive in time phase, while the direct waves have a phase difference. The windings Vfl; and lV.l of the coil C2 are so arranged that the outputs ot' the intermediate frequency amplifiers are reversed. The result is that the indirectly received signals are cancelled out, and the output of the system is a vector anni oi the directly received signals, the angle determining the phase difference of the signais directly received in the two receivers varying with the distance 92.

lt is to be understood that various changes trein the disclosure of the drawing may be made within the scope of the appended claims without a departure from the spirit of the invention.

What is claimed is:

1. The method of receiving radio signals which follow both a direct path and an indirect path in a single vertical plane between the point of transmission and any point of reception, which consists in receiving the direct and indirect signals from a single source at tivo points in a common vertical plane with the point ot transmission, said points being so chosen that the signals received at the two points over the one path will be in phase and the signals received over the other path will have a phase difference, and combining the outputs from said two oints so that the signals received over the rst of said paths will cancel one another, and the signals received over the other of said paths will have a positive vector sum.

A system for receiving radio signals which follow both a direct path and an indirect path in a single vertical plane between the point of transmission and any point of reception, said system comprising two radio receivers substantially in a common vertical plane with the point of transmission and so disposed in said plane that the signals received in the two receivers over the one path will be in phase and the signals received over the other path will have a phase diderence, means for combining the outputs of the two receivers, and means for cancelling the signais received over the first path.

Il. A system for receiving radio signals which follow both a direct path and an indirect path in a single vertical plane between the point ot transmission and any point of reception, said system comprising two radio receivers substantially in a-common vertical plane with the point of transmission and so disposed in said plane that the signals received in the two receivers over the one path will be in phase and the signals received over the other path will have a phase ditference, means for combining the outputs of the two receivers, means for cancelling the signals received over the iirst path, a detector, and means for impressing the vector sum ot the signals received over the second path on said detector.

in testimony whereof, I have signed my name to this specilication this 2nd day of December, 1926.

RALPH K. POTTER. 

